1. Technical Field
The present invention relates to an image display apparatus, and more particularly to an image display apparatus which includes a double light modulation element structure having a first light modulation element and a second light modulation element.
2. Related Art
Recently, improvement over image quality of electronic displays such as LCD (liquid crystal display), EL (electro-luminescence) display, plasma display, CRT (cathode ray tube), and projector has been remarkable, and devices having high performance substantially equivalent to vision characteristics of humans in view of resolution and color range are under development. However, according to these displays, reproducibility of the luminance dynamic range is only in the range from about 1 to about 102 [nit], and the typical gradation number is 8 bits. As for the vision of humans, however, the luminance dynamic range of recognition at a time is about 10−2 to 104 [nit], and the luminance distinction capability is 0.2 [nit], which corresponds to 12 bits when converted into gradation number. When display images on a display apparatus currently available are viewed through these vision characteristics, the narrowness of the luminance dynamic range of the device is emphasized. Moreover, the gradations of shadows and highlights are insufficient. Thus, the viewers are not satisfied with the images on the display in view of reality and power of the images.
In case of CG (computer graphics) used in movies, games and the like, the movement of pursuit for reality of description by giving a luminance dynamic range and gradation characteristics close to the vision of humans to display data (hereinafter referred to as HDR (high dynamic range) display data) is coming to the mainstream. However, the ability of expression the CG contents originally posses cannot be fully offered due to insufficient performance of the display apparatus used to provide the description.
In addition, the OS (operation system) coming next will adopt 16-bit color space, which greatly increases the dynamic range and gradation number from those of the 8-bit color space currently used. It is thus estimated that demand for such an electronic display apparatus which can provide sufficient dynamic range and gradation number for utilizing the 16-bit color space increases.
In various types of display apparatus, projection type display apparatus (projectors) such as liquid crystal projector and DLP (digital light processing, trademark) projector can provide particularly large screen display, and therefore is an effective display apparatus for reproducing reality and power of display images. For overcoming the above drawbacks, the following technology has been proposed in this field.
According to the basic structure adopted in a projector which enlarges luminance dynamic range in this technology, light emitted from a light source is modulated by a first light modulation element to create desired illumination light quantity distribution. This illumination light quantity distribution is transmitted onto a second light modulation element. After further modulated by the second light modulation element, illumination is provided. Each of the light modulation elements is constituted by a transmission type light modulation element which has a pixel structure or a segment structure whose light propagation characteristics can be independently controlled and controls two-dimensional transmittance distribution. A liquid crystal light valve is a typical example of the transmission type light modulation element. JP-A-2005-250440 discloses an example of the liquid crystal light valve. According to this example, a relay lens is disposed on an optical path between the first light modulation element and the second light modulation element, and an optical image of the first light modulation element is supplied to the light entrance surface of the second light modulation element via the relay lens. Alternatively, a reflection type light modulation element may be used instead of the transmission type light modulation element, and a typical example of the reflection type light modulation element is a DMD (digital micromirror device) element.
It is now assumed that a light modulation element used has transmittance of 0.2% for dark display and transmittance of 60% for bright display. A related-art projection type display apparatus uses this light modulation element as a single unit, and the obtained luminance dynamic range is 60/0.2=300. On the other hand, the display apparatus discussed above has a structure where two units of this light modulation element having the luminance dynamic range of 300 are disposed optically in series. In this case, the luminance dynamic range of 300×300=90000 can be achieved theoretically. The same theory is applicable to the gradation number. By disposing the light modulation elements each of which has 8-bit gradation number optically in series, the gradation number larger than 8 bits can be obtained.
The first light modulation element and the second light modulation element are separately driven by a first modulation signal and a second modulation signal produced from an image signal, respectively.
According to the projection type display apparatus disclosed in JP-A-2005-250440, the optical image of the first light modulation element is supplied to the second light modulation element by using the relay lens. In this case, deviation or distortion of image-formation magnification is caused on the optical image of the first light modulation element due to optical aberration and manufacture error of the relay lens or for other reasons, or discrepancy of alignment between the optical image and the second light modulation element. As a result, the grid structures of the pixels contained in the first light modulation element and the second light modulation element interfere with one another, thereby causing moiré effect and considerably deteriorating the display image quality. For reducing generation of moiré effect, highly accurate relay lens and alignment mechanism need to be equipped, which increases the cost of the apparatus.
In order to overcome these drawbacks, a structure eliminating the relay lens between the first light modulation element and the second light modulation element is considered. Even in this case, the optical image of the first light modulation element is projected on the screen, and thus light emitted from the first light modulation element is subject to diffraction while passing through the second light modulation element. As a result, the projection image on the screen becomes a multiple image, which lowers the display image quality. Moreover, fine luminance control over each pixel cannot be performed when the relay lens is eliminated, and therefore precise image expression cannot be achieved.